The invention relates to a method for controlling the deflection and/or position of a deflection-compensated doctor blade support beam.
This kind of method is used for controlling the deflection and/or position of a deflection-compensated doctor blade support beam relative to a web such as a paper or paperboard web.
Paper and similar web-like material are coated by applying to the moving web of the base material a layer of coating mix which is then spread into an even layer onto the web surface with the help of a doctor blade. In the coater, the web-like material to be coated passes through a gap formed between the doctor blade and a suitable backing member, conventionally a rotating roll. The blade doctors excess coating away from the web surface and levels the coating mix into an even layer on the web surface. In order to achieve a coat layer as even as possible, the linear force loading the doctor blade against the web should be sufficiently strong and constant over the entire cross-machine length of the blade to attain uniform spreading of the coating mix onto the web even at high web speeds.
For several reasons, the force loading the doctor blade against the material web does not stay exactly constant. During its machining, the doctor blade and its frame are fixed to the machining unit base with strong fixtures into a position simulating their operating position. Despite exact placement of the fixtures on the machining unit, defects will develop during the machining of the doctor blade and its frame causing an error to appear in the parallel alignment between the web surface and the doctor blade tip. As the doctor blade of the coater is pressed against the moving web, the blade is loaded with a linear force. However, due to the pivotal support of the doctor blade frame by bearings mounted at both ends thereof, the deflection induced by the linear load force becomes greater at the center of the blade than at the supported ends, whereby the blade runs closer to the web at its ends than at its middle portion. Since the linear force exerted by the blade onto the surface of the web or the backing roll is smaller in the middle of the blade than at its supported ends, the profile of the applied coat becomes uneven.
Calenders are today equipped with deflection-compensated rolls rotating about a load-bearing center shaft roll. Between the center shaft roll and the roll shell surrounding the same are adapted compensation elements whose shape can be controlled so as to keep the roll shell straight in a cylindrical shape. In U.S. Pat. No. 5,269,846 is disclosed a doctor blade support beam comprising a box-section frame, together with a holder of the doctor blade, and a support tube placed to the interior of the frame. The support tube is backed against the frame by means of three asymmetrically placed compensating elements that advantageously are pressurized hoses. The deflection of the doctor blade beam is compensated for by varying the volume of the compensating elements through pressure alterations in the elements. With the help of three compensating elements, the doctor blade position can be adjusted in desired direction in the cross-sectional planes of the doctor blade support beam. By virtue of the thus accomplished position shift, the deflection of the doctor blade can be compensated for up to an essentially perfect straightness. The compensating system is controlled with the help of a feedback control loop using data obtained from a direct measurement of beam deflection, or alternatively, from the surface profile of the coated web. The straightness of the beam is controlled on the basis of measurement data either automatically or manually.
In conventional control methods developed for a deflection-compensated doctor blade support beam, automatic control is accomplished by way of, e.g., first adjusting the center point of the support beam, or any other suitable reference point, to a desired position by changing the temperature of the thermal compensation circuit elements of the doctor blade support beam based on the desired direction toward which the center point of the support beam should move. Each thermal compensation circuit can move the position of the support beam center point and, thus, the beam deflection, in the working direction toward which the respective compensation circuit is adapted to effect. Herein, the term working direction must be understood as the direction of support beam movement under the effect of a temperature change induced in the thermal compensation circuit. The working directions of the thermal compensation circuits can be determined either by mechanical modeling computations or by effecting temperature changes in each thermal compensation circuit separately and then determining therefrom the magnitude and direction of the induced response.
A problem hampering this prior-art technique is that the control of the support beam response is slow. This is because a slightest control action requires a change in the temperature of the heat transfer circuit. As a result, the settling time of the control system defined, e.g., as the response time (within a preset tolerance) from the launch of a control command to the instant the desired blade position is attained becomes longer. Furthermore, if only two separate thermal compensation circuits are used, the deflection of the doctor blade support beam can be adjusted only in regard to one linear control line. Herein, it is possible that the control line is least optimal as compared to the desired direction of control.